Participation of nonsegmentary neurons of the spinal cord in execution of presynaptic control

Abstract

Experiments were carried out on cats six days after complete transection of the spinal cord. Cord dorsum potentials (CDP) were recorded in the vicinity of the third lumbar segment during stimulation of the isolated dorsolateral funiculus (DLF). The CDP consist of a rapid monophasic potential (which apparently reflects antidromic excitation of the cells of Clarke's column) and two subsequent slow negative waves, which are replaced by a long positive oscillation. In form, time characteristics, and behavior during thythmic stimulation, this potential differs considerably from the CDP recorded during stimulation of the afferent nerves. The presence of a positive phase of the CDP indicates that stimulation of the DLF evokes primary afferent depolarization (PAD). Stimulation of the DLF causes inhibition of the CDP evoked by stimulation of the afferent nerve. The time course of this inhibition corresponds to the time course of presynaptic inhibition. It is demonstrated that stimulation of the afferent nerve (n. femoralis) inhibits slow components of the CDP evoked by stimulation of the DLF. This inhibition reaches a maximum at the 16th millisecond; its duration exceeds 300 msec. Stronger and more prolonged inhibition of the same components is observed when both the conditioning and the testing stimuli are administered to the DLF. Since primary afferents do not take part in CDP emergence during stimulation of the DLF, it may be hypothesized that the observed inhibition develops as a result of depolarization of interneuron axon terminals.